Cryptands flexibility

An even more complicated nomenclature problem arises with the closely related all-oxygen cryptands. These compounds do not utilize nitrogen as the three-chain junction. Most examples of this class of compounds have utilized pentaerythritol or glycerol as the junction. This naturally imparts a somewhat lower flexibility to the molecule than would be present in the nitrogen-containing cases. Structures of two such molecules are illustrated below. 

In general, the cryptands (213) show a stronger correlation between thermodynamic stability and match of the metal ion for the cavity. Thermodynamic data for complexation of the alkali metal ions with a number of cryptands is summarized in Table 6.2. The data for the smaller (less flexible) cryptands 2.1.1, 2.2.1, and 2.2.2 illustrate well the occurrence of peak selectivity. 

In contrast to the peak selectivity just discussed, there is evidence that the larger, more flexible, ligands tend to exhibit plateau selectivity - a reflection that a number of the larger metal ions are accommodated by the cryptand without major variation in binding energy. 

Solid-liquid phase-transfer catalyst.1 The reagent represents a new class of catalysts, acyclic cryptands or tridents. It is singled out of a group as the best compromise of efficiency/price/toxicity. It solubilizes salts of alkali metals as well as of transition metals such as RuC13 and PdCl2, probably because of the flexibility of the molecule. In addition the trident is sensitive to the nature of the anion, but anionic activation is less than that obtained with cryptands. 

Thus K+ is bound selectively compared to Na+ by 18-crown-6 and [2.2.2]cryptand, while the reverse is true for dicyclohexyl-16-crown-5 and [2.2.1]cryptand. These ligands are shown in Figure 3. The cation Na+ misfits in 18-crown-6 due to its smaller size. The cryptands form complexes of greater stability than do the crowns, and also show greater selectivity as they have a three-dimensional cavity. These ligands have a flexible structure that allows the stepwise replacement of aqua groups on the metal ion. 

The cryptands are sufficiently flexible to bind very different cation sizes despite some considerable deviations of the —CMpO— and —C- -C— torsion angles from the preferred values of 180 and 60°. 

The kinetics and dynamics of crvptate formation (75-80) have been studied by various relaxation techniques (70-75) (for example, using temperature-jump and ultrasonic methods) and stopped-flow spectrophotometry (82), as well as by variable-temperature multinuclear NMR methods (59, 61, 62). The dynamics of cryptate formation are best interpreted in terms of a simple complexation-decomplexation exchange mechanism, and some representative data have been listed in Table III (16). The high stability of cryptate complexes (see Section III,D) may be directly related to their slow rates of decomplexation. Indeed the stability sequence of cryptates follows the trend in rates of decomplexation, and the enhanced stability of the dipositive cryptates may be related to their slowness of decomplexation when compared to the alkali metal complexes (80). The rate of decomplexation of Li" from [2.2.1] in pyridine was found to be 104 times faster than from [2.1.1], because of the looser fit of Li in [2.2.1] and the greater flexibility of this cryptand (81). At low pH, cation dissociation apparently... 

The earliest molecules designed specifically to encapsulate other species were of a type now known as cryptands, including sepulchrates and sarcophagenes, and have been described in Chapter 1. These compounds make use of two bridgehead atoms, often carbon, nitrogen or boron, which are linked by three molecular strands. These molecules are intentionally quite flexible to allow small guests to enter but, once inside, donor atoms within the strands surround the guests with numerous weak... 

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